Cytochrome P450 enzymes catalyze key steps in the biosynthesis of cholesterol and all other steroids, the metabolism of most drugs and xenobiotics, and the undesirable conversion of xenobiotics to carcinogens and toxic species. The long term objectives of this project are to define the active site structures of cytochrome P450 enzymes, to elucidate their catalytic mechanisms, to develop isozyme-specific, mechanism-based inactivating agents for them, and to use the inactivating agents to explore the biochemistry and physiology of the cytochrome P450 system. A further major goal is to develop a general theory of hemoprotein function. The immediate aims of the project are to (a) unambiguously establish the structures of the various classes of cytochrome P450 complexes that may have a carbon atom bound to the heme iron atom, (b) use the heme complexes to position photolabels in the active sites of purified cytochrome P450 isozymes (notably P450b, P450e, P450c, P450d, and P450LAw) in order to obtain information on the protein sequences that define their active sites, (c) use heme complexes and N- alkyl heme adducts to define the topologies of the substrate binding and catalytic sites of individual cytochrome P450 isozymes, (d) better define the catalytic mechanism of cytochrome P450, in particular the mechanism of pi-bond oxidation and the reaction of the enzyme with dialkylperoxides, (e) continue the development of isozyme-specific inactivating agents with a particular emphasis on fatty acid, prostaglandin, and eicosanoid w-hydroxylases, and (f) continue to use mechanism-based inactivating agents to determine the biological roles of cytochrome P450 enzymes.